This invention relates to rotary head recording and/or reproducing apparatus of the type which records/reproduces tracks containing an information signal area and a track following area, the latter being used in a reproducing operation to control the tracking of a head as it scans a track and, more particularly, to such apparatus wherein timing sync data is recorded in the track following area for the purpose of establishing a precise after-record area for the after-recording of additional information in the track.
Rotary head recorders have long been used to record video signals and, more recently, to record digital audio signals, the latter being recorded on digital audio tape (DAT). In such rotary head recorders, information, such as video information, audio information or multiplexed video and audio information are recorded in an oblique track across a record tape or other record medium and such information may be recorded in analog or digital form. Digital recorders of the rotary head type generally provide superior performance characteristics because of the inherent advantages derived from digital recording techniques, namely the accuracy with which a recorded digital signal may be reproduced and the ability to rely upon digital error correction techniques, such as error correction codes, to compensate for errors that, nevertheless, may be introduced during the recording or reproducing operation. A typical digital video recorder, as an example, records at least three types of information in a track: a digital video signal, a digital audio signal and a digital subcode signal which may be used as a control signal, such as a display control signal. In such digital video recorders, the video, audio and subcode information typically are recorded in time division multiplexed form such that the video information is recorded in a video section, the audio information is recorded in one or more audio sections and the subcode information is recorded in a subcode section in a track. In addition, and to assure reliability during a reproducing operation, it is common to record a pilot signal in one or more additional areas in the track, such areas being designated automatic track following (ATF) areas.
In one proposal for a digital rotary head recorder, ATF areas are disposed in advance of the information area (i.e. an ATF area is scanned before audio, video and subcode information is scanned) and also in areas which follow the information area. As a rotary head rotates into scanning relationship with a track, the head begins its scan at a head entering end of the track and the head rotates out of its recording relation with the track at a head leaving end. ATF areas are recorded in the vicinity of the head entering end and the head leaving end, respectively. The pilot signals which are reproduced from the head entering end ATF area and from the head leaving end ATF area are used in a servo loop to adjust the speed at which the tape is transported, thereby assuring that the heads are generally centered over the 4 respective tracks which they scan.
It often is desirable to operate rotary head recorders of the aforementioned type in an after-recording mode. An after-recording mode also is known as a dubbing mode wherein audio information, for example, is after-recorded in a track at a time subsequent to the recording of video information in that track. Such audio information may be a translation of audio signals from one language to another, thereby "dubbing" such audio information in synchronism with the movement of, for example, a subject's lips. As another example, the audio information may be in the form of a "voice over" but, nevertheless, is related to the video information. Such voice over techniques are well known and are commonly used in video production, programming and the like. In addition to after-recording audio information, as just described, it also is common to record video information or even subcode information in an after-recording mode. In these after-recording operations it is important to define with accuracy the particular area in a track in which the audio or video or subcode information is to be after-recorded. For example, if audio information is to be after-recorded, it is important that such audio information be recorded only in the allotted audio sections so as to avoid inadvertent erasure or "over-writing" of video information. Likewise, if video information is to be after-recorded, it is important that the beginning and end of the video signal area be defined accurately so that the after-recorded video information is recorded substantially only in the video area and does not erase previously recorded audio or subcode information. Likewise, if subcode data is to be after-recorded, it is important that the subcode area be defined accurately so that subcode data is recorded substantially only in that area.
Since ATF areas are recorded in preceding and following relation with respect to information areas (it will be understood that the term "information area" is intended to refer to one or more, or even all, of the audio, video and subcode areas), it had been thought that, in an after-recording mode, the pilot signal normally recorded in the ATF area can be detected and used to define the appropriate after-recording area. However, the pilot signal typically is nothing more than a burst of pilot frequency. If that burst is detected at the beginning of the ATF area and used as a reference from which the after-recording area is established, it is expected that the after-recording area will properly begin at the very beginning of the information area. But, if the pilot burst is not detected until the middle or even the end of the ATF area, then the beginning of the after-recording area may be somewhat delayed. This delay often is quite variable and unpredictable. Consequently, the after-recording area may begin too late, thus leaving previously recorded information which should have been erased (or overwritten); and the after-recording area may extend into a section of the track which should not have been erased. Thus, by relying upon the accurate detection of a pilot signal, the precise definition or establishing of the after-recording area generally cannot be carried out with a high degree of accuracy. Hence, desired signals which should not have been erased by after-recording may, in fact, be over-written. This problem is exacerbated by the fact that the frequency of the pilot signal normally recorded in the ATF area is quite low, which means that the pilot signal might not be accurately detected until a substantial portion of the ATF area has been scanned.